In an attempt to understand the genetic control and molecular mechanisms of sister chromatid exchange in the yeast Saccharomyces cerevisiae, mutants with elevated levels of sister chromatid exchange have been isolated. These mutants were isolated in a strain that is deleted for the resident HIS3 gene and that contains two truncated copies of the HIS3 genes integrated near the centromere of chromosome IV; unequal sister chromatid exchange between homologous regions of the HIS3 gene can restore the functional gene. Mutants elevated for sister chromatid exchange were identified based on their increased ability to form HIS+ prototrophs compared to the control unmutagenized clones. Three mutants (esr1, esr2, esr3) have been characterized extensively. Each of these mutants behaves like a single Mendelian locus. esr1 and esr2 have elevated levels of mitotic gene conversion. Experiments are underway to clone the esr1, esr2, and esr3 genes by complementation. Using the same system, effects of know RAD, mutations, meiosis, and DNA damaging agents on SCR have been measured. We are also determining the influence of various repair genes on SCR.